Denali

National Park Service U.S. Department of the Interior Denali National Park & Preserve geology road guide Denali National Park & Preserve geology road guide Capps . McLane . Chang . Grover . Strand Credits Former Denali geologist Phil Brease initiated this guide for the benefit of countless future staff and visitors. It is the product of multiple years’ worth of efforts by National Park Service (NPS) staff and Geological Society of America (GSA) interns. Authors Denny Capps, NPS Geologist, Denali National Park and Preserve Sierra McLane, NPS Director, Murie Science and Learning Center Lucy Chang, GSA Intern, Geoscientists-in-the-Parks program Layout and Design Ellen Grover, NPS Science Communicator, Denali National Park and Preserve Sarah Strand, GSA Intern, Geoscientists-in-the-Parks program Laura Vachula, NPS Education Technician, Denali National Park Preserve Special Thanks Christina Forbes, GSA Intern, Geoscientists-in-the-Parks program Chad Hults, Don and Sandy Kewman, Jan Tomsen, Kara Lewandowski, Ron Cole, and other contributers and reviewers Cover Photo NPS Photo / Tim Rains How to Cite This Book: Capps, D., McLane, S., and Chang, L. 2020. Denali National Park and Preserve Geology Road Guide (3rd ed.). National Park Service, Denali National Park and Preserve, Denali Park, Alaska. Available for free online. The original edition of this book was inspired by the 2016 National Park Service Centennial during which Denali celebrated 100 years of geological research and exploration. C016245 Dedication This guide is dedicated to Phil Brease (Denali Park Geologist from 1986 to 2010). Phil’s humor, wisdom, music, and love of adventure and geology will never be forgotten by the many people whose lives he touched. Throughout his years as Denali’s geologist, Phil discovered fossils, monitored glaciers and road hazards, reclaimed mined lands, taught geology, and predicted that if we kept looking in the right places, someday we would find a dinosaur track in the park. Denali geology is no piece of cake, or one should say ‘layer cake,’ as places like the Grand Canyon are often described. In fact, I like to describe the geology of Denali as a mix of several well-known western parks. The recipe is to place the sediments of the Grand Canyon, the plutonic rocks of Yosemite, and the volcanics of Mount Rainier in a blender, and turn it on briefly to ‘chop.’ Then layer as a parfait, and serve with large quantities of ice from the likes of Glacier Bay National Park! —Phil Brease NPS Photo Table of Contents Part 1 From Old Rocks to Young Ice: Entrance Area to Teklanika Part 2 1 Dynamic Denali: Teklanika to Toklat 29 Mount Healy 6 Where Teklanika and Cantwell Formations Meet 32 Glacial Forces 7 Teklanika Dikes 33 Glacial Erratics 8 First Dinosaur Footprint Found In Denali 36 Drunken Trees 9 Tattler Creek 37 Hines Creek Fault Expression 10 Igloo Creek Debris Slide 40 Lignite/Dry Creek Terminal Moraine 11 Sable Pass Debris Slide 42 The High One Emerges 12 Coal Mining by the East Fork Toklat 43 Gossan 13 Polychrome Pass 44 Gravel Ridge 14 Bear Cave Slump 46 Building the Park Road 47 Pretty Rocks Landslide 48 Spheroidal Weathering 50 Polychrome Overlook—Looking North 51 Savage River 16 Nenana Gravel 18 Double Mountain 19 Antecedent Stream 23 Polychrome Overlook—Looking South 52 Drunken Forest 24 Patterned Ground 55 Kettle Ponds 25 Road-Blocking Debris Flows 58 Teklanika River and Surrounding Area 28 Toklat River 59 Part 3 A Park of Unusual Scale: Toklat to Kantishna Geo Features 60 Rock Types 4 Highway Pass 64 Braided Rivers 15 Bergh Lake 65 Aufeis 22 Denali Unobscured 67 Assembling Alaska: Accreted Terranes 26 Eielson Visitor Center 68 Paleontological Wonders 34 Pillow Basalt 71 Precarious Permafrost 38 Muldrow Glacier 72 Glaciers 56 Hines Creek Fault 74 Why is Denali so Tall? 70 Muldrow Moraines 75 Earthquakes 78 Western Kettle Ponds 76 Geologic Timeline 84 Wonder Lake 77 Glossary 85 Glaciofluvial Terraces 80 References 88 Seismic Activity in the Kantishna Hills 81 Index 94 Kantishna Area Mining Legacy 82 Introduction Denali National Park and Preserve is a place where powerful geologic forces—tectonics, volcanism, and glaciation, among others—have collectively produced a stunning showcase of landscape features. Some features dominate, like the flanks of Denali and the glacially-carved valleys that surround it, while others may only be noticed by the trained eye. This guide highlights some of the most interesting geological phenomena that can be experienced from the Denali Park Road. It stands on the shoulders of several past road guides, including some by the previous park geologist, Phil Brease10,48. Though the text is composed with an east-to-west drive in mind, each feature stands alone, allowing for the guide’s use regardless of where you are or how you got there. If you are new to the field of geology or to Denali, you may want to read the GEOFeatures first, as they cover broad topics such as rock types, how Alaska and the Alaska Range were formed, braided rivers, paleontology, glaciers, and earthquakes. Skimming the glossary will also make the text more understandable for those unfamiliar with geology terminology. For readers with physical science backgrounds, be aware that some geologic names (terranes, glaciations, and such) are capitalized in non-conventional ways to aide comprehension for the layperson. The text is divided into three sections that group geologically-similar areas and roughly divide the Denali Park Road into thirds. Each section opens with an overview map and contains mile-by-mile feature descriptions, GEOFeatures, and fun facts. Through this book, we invite you to celebrate 100 years of geologic research and exploration in Denali National Park and Preserve. In 2016 and 2017, the National Park Service and Denali celebrated back-to-back Centennials. Both birthdays honor the researchers and rangers who have dedicated themselves to understanding and stewarding America’s special places. We hope you enjoy exploring Denali geology through these pages and in person, and by doing so, help us launch another 100 years of learning and conservation. NPS Photo / Tim Rains PART From Old Rocks to Young Ice 1 Park Entrance to Teklanika For the first 30 miles, the Denali Park Road parallels the Hines Creek Fault while winding between the Outer Range to the north and the foothills of the far larger Alaska Range to the south. This portion of the road showcases geologic wonders ranging from the oldest rocks in the park to modern permafrost features. The Outer Range is composed of rocks that have been metamorphosed by multiple episodes of compression and heat. Much more recently, glaciers carved the valleys through which the road winds, transported huge amounts of debris, and left behind erratics and other evidence of their passage. While these particular glaciers are now gone, the ground is still permanently frozen beneath portions of the taiga and tundra ecosystems through which you will soon travel. However, climate change is starting to thaw Denali’s frozen ground, with tangible impacts on trees, roads, and infrastructure. 1 Mile Feature 0–15 Mount Healy 2.4 Glacial Forces 3–5 Glacial Erratics 3–8 Drunken Trees 7.1 Hines Creek Fault Expression 8.3 Moraine of Lignite/Dry Creek Glacial Advance 9–13 The High One Emerges 13.5 Gossan 13.9 Gravel Ridge 14.7 Savage River 18.6 Nenana Gravel 19.7 Double Mountain 22.8 Antecedent Stream 23.5 Drunken Forest 28.8 30.2 Kettle Ponds Teklanika River and Surrounding Area 22 21 23 25 26 27 28 29 30 31 32 33 2 24 18 20 19 8 1 2 15 3 14 17 16 13 12 11 10 8 9 3 7 6 5 4 0 GEO FEATURE Rock Types Three major types of rocks make up Earth’s crust: igneous, sedimentary, and metamorphic. Igneous rocks are formed when molten rock solidifies. When magma cools slowly deep within the earth, the resulting rocks—granite being a classic example—are classified as intrusive. When magma cools rapidly at or near the surface, often as lava emerging from a volcano, it is classified as extrusive. Basalt, rhyolite, and andesite are common types of extrusive igneous rocks. All igneous rocks are characterized by crystalline structures. Intrusive rocks tend to have large crystals, while extrusive rocks have small, often indiscernible crystals. Igneous rock formations described in this guide include the Teklanika Formation and Mount McKinley Granite. Below: Plant fossils in a sedimentary rock tell us about the park’s past. NPS Photo NPS Photo / Tim Rains Above: Igneous rocks give Polychrome Pass its iconic multi-hued tones. Sedimentary rocks are derived from sediments—particles of mineral and organic material that have been deposited by water or wind. Sediments are commonly carried by rivers or streams and deposited in basins such lakes and seas. These sediments are often buried and compressed into rock. Typical sedimentary rocks 4 include sandstone, limestone, shale, and chert. Fossils found in sedimentary rocks provide clues to the environment in which the sediment was deposited. Sedimentary rock formations described in this guide include the Cantwell Formation, Kahiltna Flysch, the Usibelli Group, and Nenana Gravel. Metamorphic rocks are preexisting rocks that have been changed due to intense heat and/or pressure without completely melting. The original rocks can be sedimentary, igneous, or even metamorphic. When rocks are squeezed and baked beneath Earth’s surface or by contact with magma, minerals may be replaced or the rock may change texture. Typical metamorphic rocks around Denali include schist, slate, and marble. The Yukon-Tanana Terrane is the only metamorphic group of rocks described in detail in this guide. Four other terranes found in Denali (see p.26) also contain metamorphic rocks. NPS Photo / Tim Rains 5 MILE Mount Healy 0–15 The park entrance area is dominated by Mount Healy, which looms just north of the park road. Though Mount Healy is referred to as one mountain, its ridgeline actually extends for 15 miles (24 km) from the George Parks Highway to the Savage River. Mount Healy is part of the informally named Outer Range, which also includes Mount Margaret and Mount Wright to the west of the Savage River. The foundation for the entire Outer Range is the metamorphic Yukon-Tanana Terrane. Terranes are fragments of the Earth’s crust that have been scraped or broken off of one tectonic plate and sutured onto another. The rocks of this terrane, the oldest that you’ll find in the park, formed ~400 million years ago as shallow-water seafloor deposits and were intruded by igneous rocks ~365 million years ago23,29. In the Early Jurassic Epoch (~195 million years ago), these rocks were accreted onto the North American continent. Since then the Yukon-Tanana rocks have experienced multiple episodes of regional metamorphism3, 45. Photo by Andrew Collins 6 The Yukon-Tanana Terrane is the foundation for roughly the northern third of the park and many of the nearby mountains that line the George Parks Highway, including Sugarloaf Mountain northeast of the park entrance. The Yukon-Tanana Terrane extends northwards to the Brooks Range and makes up the hills surrounding Fairbanks, as well as east into Canada56. Below: The Mount Healy ridgeline provides breathtaking views of the park. MILE Glacial Forces 2 . 4 Look around and imagine the vast amounts of ice—thousands of feet thick and miles long—required to shape the landforms that you see ice-free today. As recently as 22,000 years ago, much of this area would have been covered in glacial ice. During the last ice age, glaciers advanced north from the Alaska Range into the current Nenana River valley during four major glaciations: the Teklanika Glaciation ~2.8 million years ago72, the Browne Glaciation during the mid-Pleistocene (numerical age unknown, but >300,000 years ago27), the Lignite/Dry Creek Glaciation ~500,000 years ago44, and the Healy Glaciation ~65,000 years ago27,14. From this bend in the road, look east past the railroad trestle to where the Nenana River flows in the distance. Approximately 22,000 years ago, the much younger and smaller Riley Creek Glaciation would have terminated here14. The terminal moraine still lies immediately to the right (south) of the train trestle, but is largely obscured by vegetation. The Riley Creek Glaciation was likely a re-advance of the more extensive Healy Glaciation80. The Healy Glaciation is named after a terminal moraine south of the town of Healy. As the glacier receded, meltwater that was dammed behind the Healy moraine and a bedrock ridge formed the 400-foot (122 m) deep, 11-mile (18 km) long prehistoric Lake Moody in the valley between Healy and the park entrance80. Lake Moody has long since drained, but evidence of its existence is still apparent in unstable landforms that cause severe problems for the railroad, highway, and buildings in the Nenana Canyon. A lobe of the Healy glacier flowed uphill from the main north-south valley, depositing the till (glacial sediment) that Park Headquarters at Mile 3.4 is built on. Below: Map of glacier extents around the park entrance during the last ice age80. Terminal Moraine of Riley Creek Glaciation Denali Park Road Lignite/Dry Creek Glaciation Glacial Lake Moody Healy Glaciation Riley Creek Glaciation Browne Glaciation 7 MILE Glacial Erratics 3 – 5 The rock next to the sign for Park Headquarters (Mile 3.4) is a foreigner in Denali. It’s a glacial erratic—a rock transported by a glacier and often, but not necessarily, made of material exotic to its surroundings. You can sometimes track an erratic’s origin based on its mineral composition. Photo by Lian Law On nice days you can see two more erratics at the top of the hill to the south behind Park Headquarters. Notice how unusual they are in both size (more than 30 feet [10 m] wide) and shape compared to their smooth, glacially-scoured surroundings. Upon closer inspection, you’d notice that the erratics are granitic. 8 However, the closest bedrock outcrop of granite is many miles away. This means that during the Dry Creek Glaciation several hundred thousand years ago, these rocks traveled many miles on a lobe of ice and were dropped in their present locations when the ice beneath them melted80,11. MILE Drunken Trees 3 – 8 In the forests on both sides of the road, you may notice a few leaning trees. Originally upright, some of the trees in this area are now tipping over as a consequence of thawing permafrost. The trees are rooted in a shallow top layer of soil that thaws seasonally; this soil is referred to as the ‘active layer’. When the permafrost underneath the active layer thaws, the trees have very little root depth to help them remain upright on the newly unsupported ground. The result is that the trees look like they’ve had a little bit too much fun. Drunken forests (a real scientific term!) are often found by roadsides or lakes, where heat is more easily transferred into the frozen ground. Throughout the park, black spruce is the tree species most typically found in drunken forests because it is tolerant of the watersaturated soils often found on top of permafrost. While you may see a few leaning trees here, Mile 23.5 near the Sanctuary River provides a better example of a drunken forest. Right: Tipsy trees occur intermittently along the park road to the west of Park Headquarters. NPS Photo / Denny Capps 9 N MILE Hines Creek Fault 7.1 Here the road crosses Hines Creek and travels parallel to a regionally-important geologic feature, the Hines Creek Fault. This fault is an active part of the larger Denali Fault system that arcs east-west across the state. While the fault gets its name from the creek, the creek itself is a fault-controlled drainage, meaning that it follows the path of the fault and probably wouldn’t exist if it weren’t for the weakened material caused by tectonic activity. The Hines Creek Fault trends between the park road and the Alaska Range foothills to the south, acting as a boundary between two terranes—the Yukon-Tanana Terrane to the north and the Pingston Terrane to the south69. Above: The Hines Creek Fault appears as a dotted red line just south of the Park Road on this annotated USGS topographic map69. However, glaciations, landslides, stream deposition, and other geologic processes have left the fault largely invisible here. Near the park entrance, visible but out of view from the park road or any trail, lies a recent (~1,300-year-old) scarp of the Hines Creek Fault32. This specific fault scarp remained unnoticed, because vegetation obscured its location, until 2011 when a LiDAR (Light Detection and Ranging) survey allowed the Earth’s surface to be seen in intricate detail. This imagery revealed that the fault trends directly underneath the northern abutments of the Riley Creek bridge on the Parks 10 Highway. Luckily this was discovered in time for the new Riley Creek bridge, built in 2015, to be designed to accommodate offset from the fault. Pa rk Ro ad Above: 2011 LiDAR image of entrance area showing Hines Creek Fault (red arrows), Park Road (green), and Parks Highway (blue). MILE Lignite/Dry Creek 8 . 3 Terminal Moraine Here the road cuts through the terminal moraine of the Lignite/Dry Creek Glaciation which flowed westerly, and uphill, from the Nenana River valley ~500,000 years ago44,80. This uphill glacial flow illustrates the tremendous mass and power of the parent glacier, from which the lobe that left behind this moraine was merely an offshoot. The moraine is not obvious from the ground because so much time has passed since its deposition. Erosion and vegetation now obscure its presence. However the feature stands out clearly on a digital terrain model, which is a 3D representation of the Earth’s surface without vegetation. It is also illustrated in the figure on page 7. Below: The terminal moraine of the Lignite/Dry Creek Glaciation is the low, thin ridge outlined by the orange dotted line on this 2010 IFSAR digital terrain model. 7 d Roa k r Pa 8 11 MILE The High One Emerges 9–13 Here you may catch your first view of Denali to the southwest if the weather is fair. White-capped and still 76 miles (122 km) away, the mountain will appear large, but not particularly taller than other nearby peaks. That’s a deception of perspective; at 20,310 feet (6,190 m) Denali is over 14,000 feet (4,300 m) taller than NPS Photo / Kaitlin Thoresen Double Mountain and the other mountains that you see in the foreground. A pullout at Mile 10.5 is the first safe place to admire the view, and Mountain Vista rest stop at Mile 12.6 is one of the better places to enjoy mountain gazing along this stretch of road. On a 12 clear day the mountain will get closer and appear bigger until you reach Wonder Lake, where the summit will only be 26 miles (42 km) away… but remember, that’s as the crow flies. It takes most of the few, hardy mountaineers who climb Denali starting from Wonder Lake each year a month to make the round trip. MILE Gossan 13.5 The orange rock outcrop on the hill just north of the road is called a gossan. The coloring is caused by oxidization of ironsulfide minerals within YukonTanana rocks—rust! Outcrops like this one oxidize into eye-catching hues when exposed to surface conditions and attract the attention of prospectors looking for gold, silver, lead, zinc, and other economically valuable resources because gossans often indicate the upper part of ore deposits. This particular area was worked by prospectors, possibly in the 1920’s and 1930’s, but was eventually abandoned13. NPS Photo / Daniel Leifheit 13 MILE Gravel Ridge 13.9 Just across the Savage River to the west is a mid-valley ridge with exposed sediment at the northern end. The genesis and evolution of this ridge and its surroundings is a point of ongoing research. Some have interpreted it to be an esker, which is a deposit from a stream running under or through a glacier11. Others have interpreted it as a remnant moraine of the Lignite/Dry Creek glacial advance from about 500,000 years ago 44,80. LiDAR data collected in 2005, which measured the topography of this area in far greater detail than any previous survey, has revealed that the gravel ridge and its surrounding features may have been truncated by tectonics after having been shaped by glaciers. Specifically, recent faulting may have caused linear striations in the stream sediments6. Ultimately these landforms are the result of several geologic processes interacting through time. The evolution of this story illustrates how modern technologies such as LiDAR are casting new light on old geologic mysteries here and elsewhere in the park. Right: Lineations (red arrow) possibly indicate tectonic shaping of the area. The exposed sediment in the photo below is located near the upper left of this digital terrain model (blue arrow). Below: The mid-valley ridge in the Savage River braidplain is visible at the bottom of the photo. NPS Photo / Kaitlin Thoresen 14 Par kR oad GEO FEATURE Braided Rivers Many of the rivers and streams in Denali are braided, consisting of many short-lived channels weaving across a wide bed of cobbles, gravel, sand, and silt. What you are seeing is not just a “dry phase”—water almost never flows over the entire width of a braided river at any one time. Braided rivers and streams occur within watersheds where relatively large amounts of sediment are transported by relatively small amounts of water. Because glaciers are such powerful agents of erosion and therefore create large amounts of sediment, braided rivers are classically, although not exclusively, glacially-fed. At higher water levels, braided rivers can transport larger amounts of sediment from upstream, but they deposit that sediment when the water level drops and slows. The deposited sediments force the water to move to new paths of less resistance. In Denali, this often happens on a diurnal basis, Below: The braids of an unnamed stream weave through Denali’s lush landscape. Photo by Diane Kirkendall 15 with higher water flows and sediment transport occurring during the heat of the day when snow and ice is melting and contributing to flow, but it also varies seasonally and in response to storms and glacial processes. Watch for river braids as you cross the Savage (Mile 14.7), Teklanika (Mile 30.2), and Toklat (Mile 53) rivers, from Polychrome Overlook (Mile 45.8), and from Eielson Visitor Center (Mile 67), among other locations. NPS Photo / Tim Rains MILE Savage River 14.7 Savage River is one of several braided rivers in Denali that are characterized by transitory channels meandering across wide beds of sediment over time. However this behavior is not the result of glacial melt, as is the case for most braided rivers in the park, because no glaciers remain in the Savage River watershed. Savage River is braided because of high sediment input from brittle rock, and from a relative lack of vegetation stabilizing its headwaters. An important transition zone that illustrates the glacial history of this area is located about halfway between the Savage River vehicular bridge and the Savage River Loop Trail footbridge to the north. Upstream (south) from this point, Savage River runs wide and shallow in a U-shaped valley typical of glacial scouring. Downstream (north), Savage River runs fast and restricted through a V-shaped valley typical of river erosion. Look for these distinctive shapes as you cross the 16 vehicular bridge11. While more extensive glaciations have affected the topography of the canyon in the past, they occurred over two million years ago27.The river has therefore had substantial time to erode the V-shaped canyon that we see today. Above: The view south from the Savage River vehicular bridge in the fall. This portion of the river flows through a valley shaped by past glaciations. Left: The view north from the Savage River vehicular bridge shows a V-shaped valley resulting from river erosion. This topography is different from the glaciallysculpted terrain seen on page 16. FUN FACT: Savage Rock (photo below), the jagged outcrop that you can walk to just uphill from the Savage River parking lot, is a surprising stumper. Why this outcrop is so prominent compared to its surroundings—despite being made of similar rocks—remains a mystery. One theory is that faulting caused the rock to protrude from the surrounding topography. A second is that it slid down from above. Or Savage Rock could be made of more resistant schist than the surrounding metamorphic rock. Further studies are needed to resolve this enigma. NPS Photo / Tim Rains The Savage River canyon is one of the few places where erosion has exposed the Yukon-Tanana Terrane bedrock along the road. Take a closer look and you’ll notice large amounts of mica-quartz schist, a shiny rock that gets its sparkle from the flaky mineral sericite and quartz veins. The varieties of metamorphic rock in the YukonTanana Terrane, along with the degree to which they have been folded and faulted, attest to the terrane’s age and dynamic history. Photo by Diane Kirkendall 17 NPS Photo / Lian Law MILE Nenana Gravel 18.6 The Alaska Range experienced the majority of its uplift in the last six million years. Scientists have deduced the timing of this impressive tectonic episode partially thanks to the humble Nenana Gravel formation that outcrops here on the north side of the road. The Nenana Gravel is a ~4,000-foot thick (1,200 m) sedimentary formation composed mostly of looselyconsolidated gravel and sand. These sediments are from igneous and sedimentary parent rocks that match the lithology (physical characteristics) of rocks in the Alaska Range. The Nenana Gravel formation is often underlain by a ~2,000-foot thick (600 m) group of sedimentary formations called the Usibelli Group, the sediments from which match parent rocks from the metamorphic Yukon-Tanana Terrane to the north61. The fact that the Nenana Gravel overlies the Usibelli Group in the Tanana Basin—a large, low area on the north side of the Alaska Range—is one of the primary ways that scientists know when the uplift of the Alaska Range began. Leaf fossils, pollen, radiometric dates, and tectonic evidence from the two formations demonstrate that the Usibelli Group sediments washed into the Tanana Basin from the north mostly during the Miocene Epoch (5–23 million years ago). Around six million years ago, these sedimentary layers started being buried by Nenana Gravel sediments flowing in from the south. This shift in sediment source indicates when the Tanana Basin was tilted north by the newly rising Alaska Range61,75. Sediment deposits continue to accumulate in the Tanana Basin today as the Alaska Range continues to rise. 18 Above: A park shuttle bus approaches a Nenana Gravel outcrop to the west of the Savage River. The summit of Denali peeks out from behind the ridge on the left. FUN FACT: The Usibelli Group of formations is mined for coal 11 miles (18 km) north of the park entrance near the town of Healy. The lignite coal dates to the Miocene and is evidence of a time when Denali’s environment consisted of swampy bogs and vast forests, a drastic departure from today’s boreal forest and alpine tundra41! The Usibelli Coal Mine is currently the only active coal mine in the state. The coal, notable for its low sulfur content, is transported to power plants in interior Alaska as well as exported to South Korea and Chile78. MILE Double Mountain 19.7 To the south lies the broad north slope of a mountain with a jagged, indistinct summit—Double Mountain. The mountain is capped by the colorful Teklanika Formation, a Tertiary (55–60 million years ago) volcanic rock unit21. The base of Double Mountain is composed of the Cantwell Formation, a late Cretaceous (~70 million years ago) sedimentary rock unit, and is intruded by the overlying Teklanika Formation21. It is noteworthy that the Teklanika Formation rocks are similar in age and mineral composition to those of the Mount McKinley Granite, which comprises many of the highest peaks in the Alaska Range, including Denali. The two rock units have very different appearances because the volcanic rocks cooled relatively quickly at the surface and therefore have a finer texture, while the granitic rocks cooled very slowly many miles deep in the crust and have a relatively coarse crystalline structure. The Teklanika volcanic eruptive center was near Denali’s peak, as demonstrated by the direction that the volcanic conglomerate flowed. This directionality is interpreted by researchers based on the orientation of clasts, or fragments, in the conglomerate. Some of the Teklanika clasts are enormous (40 feet or 12 m across), and were violently erupted out of a volcano during an event similar to the Mount St. Helens eruption of 1980 (figure p.21). Also visible is a whitish tuff, or ash layer, dipping at an angle near the upper third of Double Mountain. This 30-foot (9 m) thick tuff is the result of many thinner ash deposits accumulating over time20. NPS Photo / Russell Rosenberg NPS Photo / Russell Rosenberg Above: A sample of McKinley Granite, an intrusive rock. Above: A sample of Polychrome rhyolite, an extrusive or volcanic rock. NPS Photo / Lian Law Above: Double Mountain (left) looks shaded in the early morning light while Denali looks pinkish and hazy in the background. 19 Left: Diagrams showing the depositional and tectonic history of the Denali area before, during, and after Teklanika volcanism20. A Late Cretaceous River drainage Paleoflow Proto Alaska Range N Paleozoic rocks Cantwell Formation A) The Cantwell Formation was deposited into the Cantwell Basin on the north side of the Alaska Range ~70 million years ago (late Cretaceous) during a time of active tectonics. B) Teklanika volcanic rocks erupted from the southwest of their present location 55–60 million years ago and were deposited on top of the Cantwell Formation. B Late Paleocene Future Denali Fault Paleoflow Eruptive center magma chamber (future Alaska Range granite) Debris Avalanche Pyroclastic Flow Lava Teklanika Formation C Post-Early Eocene Denali Fault Remnant volcanoes Alaska Range granite Right-lateral offset and uplift of eruptive center 20 C) Subsequent right-lateral strikeslip movement along the Denali Fault then displaced the Teklanika rocks to the northeast relative to their eruptive center. The McKinley pluton (comprised of Mount McKinley Granite) may represent the uplifted roots of the Teklanika eruptive center. Crater Area 5 km N From Voight et al. 1981; USGS Prof. Paper 1250 Photo by Lyn Topinka NPS Photo / Lian Law Above: This large boulder (inset left) is a clast, or fragment, on Double Mountain (main photo) that is thought to have been deposited by volcanic mud that flowed out of the Teklanika eruptive center. Similar mudflows deposited huge boulders (inset right) far from the volcanic vent of Mount St. Helens during its 1980 eruption20. 21 NPS Photo / Sierra McLane GEO FEATURE Aufeis Depending on when you visit, you may see flows of ice along the park road that formed in the winter. Called aufeis or overflow, this ice usually forms in Denali when very cold temperatures freeze surface water, thereby sealing in the groundwater below. Over time (minutes to days), pressure builds up until the surface ice cracks and the groundwater flows upward onto the surface, creating a new ice layer on top. Overflow events occur repeatedly throughout Denali’s long winters, building up ice layers to depths of six feet (2 m) or more. This phenomenon is limited to high-latitude areas. While often occurring in the same places annuall